Concept
Femur
- removal of femoral head cartilage
- resurfacing with metal
- cemented / uncemented
Acetabulum
- standard technique
Bearing surface
- metal on metal
Indications
Relatively young man (40 - 50)
OA
Absolute Contra-indications
Severe bone loss femoral head
Large femoral neck cyst
Small acetabulum
Relative Contra-indications
Osteoporosis
Age > 65
BMI > 35
Caution
RA
Female
AVN femoral head
Advantages (many theoretical)
1. Bone preservation
- preserves femoral bone stock
- however makes acetabular preparation more difficult
2. Improved stress transfer to proximal femur
- less proximal stress shielding
- improved proximal bone density
3. Reduced dislocation rates
- heads 36-54 have reduced rates compared with 22-32
- can occur though if poor technique or component loosening
4. Better kinetics
- faster walking speeds
- may be better ROM
- possible better proprioception
- may be element of selection bias (i.e. is done in younger, fitter patients)
4. Easier revision of femoral component
- better bone stock
- simply recut and use stem
6. Possible improved longetivity
- very low wear rates metal on metal
Disadvantages
1. Poor modularity
- difficult to adjust LLD
- difficult to adjust offset
- patients with very abnormal abnormality better off with conventional THA
2. Not suitable for elderly / poor bone stock
- increased risk femoral neck fracture
3. Femoral neck fracture
4. Metal ions
- in serum, RBC, urine
Problems
- risk metal sensitivity
- risk carcinogenesis / teratogenesis
- CI in woman of child bearing age
5. Loosening
Complications
Femoral Neck Fracture
Incidence 0-4%
- 1.5% in a study of 3500 BHR in Australia (JBJS Br 2005)
- early in learning curve
- early in prosthesis life
Risk Factors
- decreased bone mass / osteoporosis
- elderly
- inflammatory arthritis
- females (risk x2)(AJR)
- femoral head and neck cysts
- femoral neck notching
- varus femoral component (< 130o neck shaft angle)
- cup impingement on neck
- improper implant seating
- AVN femoral Head
Revision
- relatively simple
- recut neck
- femoral implant with large metal head
Early loosening
Machining
Initially due to poor early manufacturing
- decreased clearance
- inadequate polar bearing
- increased peripheral bearing, seizing, cold welding and loosening
Modern machining
- small surface asperities
- improved fluid film lubrication
- polar bearing with small clearances
- very low wear and little particle production
Causes
A. Oversized heads / notching
B. Varus Femoral Component
C. Femoral head AVN
Due to extensive releases required to expose / surgically dislocate femoral head
D. Open Acetabular Component
Theorised to cause point loading
- increased metal wear
- best to close cup
E. Other
Australian Joint Registry 2010
13 300 procedures
Best outcome
- male < 65 with OA
- able to get > 50 mm head size
- 3.9% 9 year
Revision rate
Cumulative
- 7.2% 9 years for OA
Birmingham Resurfacing
- 6.2% 9 year
Reasons for revision
- fracture 36%
- loosening 33%
- metal sensitivity 7%
Revision by diagnosis 7 years
- OA 5.8%
- DDH 14%
- AVN 6%
Revision by prosthesis
- BHR 96.5% 5 year
Revision by age 7 years
- < 55 5.6%
- 55 - 64 5.8%
- > 65 7.3%
Revision by sex 7 years
- male 4.5%
- female 9.3%
Revision by head size 7 years
- < 44mm: 13.8%
- 45 - 49mm: 8.8%
- 50 - 54mm: 3.7%
- > 55mm: 2.2%